Freundlich Adsorption Isotherm

In the study of adsorption in chemistry, the Freundlich isotherms or Freundlich Adsorption isotherms are a graphical representation of the Freundlich adsorption isotherm equation.  This representation of the isotherm depicts the empirical relation between gas pressure and the amount of gas adsorbed into a given solid. This model of Freundlich Adsorption isotherms was created by Herbert Freundlich in 1909. Through this model, he was able to represent the isothermal variation involved in the adsorption of the amount of gas adsorbed per unit of the solid adsorbent when the pressure is constant. This relation is represented by the adsorption equation he formulated. The Freundlich Adsorption isotherms have a limitation in that they do not work in high-pressure scenarios. 

Adsorption Isotherm

An isotherm in adsorption portrays the process of adsorption and charts the relation between adsorbents and adsorbates with temperature and pressure and their variations. There are two main isotherms available in the adsorption process which are Freundlich Adsorption isotherms and Langmuir isotherm. Freundlich Adsorption isotherms depict curves showing the variations in gas adsorbed by the solid at a constant temperature and pressure. It is expressed as x/m = Kp1/n where x is the adsorbate mass, m is the adsorbent mass, n and K are constants for an adsorbent and adsorbate at a temperature. 

Freundlich Adsorption Isotherms

The Freundlich isotherms are represented in graphical formations. In this graphical representation, the amount (mass) of the gas being adsorbed is plotted against the pressure and is represented as curves on the graphs. When the pressure is fixed, there is a decrease in physisorption with a relative increase in temperature. The saturation point is depicted when the curve reaches a high point. This equation is also depicted through the log as log x/m = log k + 1/n log P. log x/m is plotted on to the y axis and log P is plotted on the x-axis. When there are solutions then the equation becomes x/m = k (C) 1/n or log x/m = log k + 1/n log C. Here, C is the concentration of adsorbate. 

                   Adsorption Isotherm based on Freundlich Equation of Adsorption

Freundlich Isotherm

The Freundlich isotherm has 1/n whose value is greater than 1 at all times. The value will always be between 0 and 1. In the Freundlich adsorption, the line plotted will be straight when the value between log P and log x/m is 1/n. 

                                       Straight slope in Freundlich Isotherm

Freundlich Adsorption Isotherm Equation

The adsorption equation of the isotherm is represented as:

• x/m = Kp1/n where x is the adsorbate mass, m is the adsorbent mass, n and K are constants for an adsorbent and adsorbate at a temperature and 1/n is essentially the value at high pressure which is 0 with p being pressure. 

• log x/m = log k + 1/n log P. log x/m is plotted on the y axis and log P is plotted on the x-axis.

• x/m = Kc1/n or or log x/m = log k + 1/n log C. Here, C is the concentration of adsorbate. 

Conclusion

The various graphical models of adsorptions help in understanding the characteristics and functions of adsorption clearly. Adsorption is applied to produce a vacuum, removing colouring materials from solutions, it is used to remove and filter impurities in gas masks and used in heterogeneous catalysts.